This application claims priority from United Kingdom Patent Application Serial No. 9916111.9, filed Jul. 10, 1999, entitled xe2x80x9cInfusion Pumpxe2x80x9d.
The present invention is directed toward volumetric infusion pumps, and more particularly toward a self-regulating servo valve for use with a volumetric infusion pump.
It is a common practice in the medical field for a therapeutic liquid to be delivered to a patient by subjecting the liquid to a known pressure and passing the liquid through a restricting device of known internal cross section to produce a reasonably constant flow rate. One manner of implementing this practice is providing and filling an elastomeric balloon with the therapeutic liquid under pressure to stretch the elastomeric balloon which, in turn, pressurizes the liquid. The liquid is then discharged through a restricting device. In practice, it has been found to be difficult to regulate the pressure on the liquid accurately over the full volume of the balloon because of the changing pressure applied to the therapeutic liquid as the elastomeric balloon changes shape during the course of an infusion. A representative apparatus for practicing this method of infusion is disclosed in Hessel, U.S. Pat. No. 4,769,008.
Another similar apparatus for infusing therapeutic liquids to a patient includes a collapsible bag containing the therapeutic liquid juxtaposed with an inflatable bladder, with the bladder and the collapsible bag being contained within a rigid housing. The bladder is inflated, for example with compressed air, to apply pressure to the collapsible bag and thus expel therapeutic liquid from it. The therapeutic liquid is expelled through an outlet tube having a fixed cross-sectional area or which includes a flow restrictor of a fixed cross-sectional area. Fluid is provided to the bladder at a controlled rate with the intent that fluid is thus forced through the outlet tube at an approximate flow rate. Representative patents disclosing this basic structure include Ross, U.S. Pat. No. Re 35,501 and McWilliams, U.S. Pat. No. 5,788,674. In practice, however, it has been found very difficult to provide a suitably constant low flow rate necessary for some clinical applications.
Shen, U.S. Pat. No. 5,749,854, attempts to overcome the deficiencies of the prior art by teaching a valve for controlling the flow of air into a bladder driving such an infusion pump. However, Shen requires a complicated and expensive valving mechanism and may still not provide a suitably constant low flow rate.
The present invention is directed toward overcoming one or more of the problems discussed above.
A first aspect of the present invention is an infusion pump including a reservoir containing a therapeutic liquid to be infused. The reservoir includes an outlet for the liquid. A volumetric air pump is operatively associated with the reservoir to pressurize the reservoir. A valve is operatively associated with the liquid outlet and prevents liquid flow through the liquid outlet until a select pressure is obtained in the outlet. The valve thereafter maintains an opening in the outlet that keeps the pressure in the reservoir at substantially the select pressure, whereby the volumetric flow rate of liquid through the outlet is proportional to the volumetric flow rate of air from the volumetric air pump. Preferably, the valve varies the opening in the outlet so as to maintain the select pressure. The outlet may consist of a flexible conduit having a select undeformed inner cross-sectional area and the valve varies the opening in the outlet by pinching a segment of the conduit to vary the inner cross-sectional area of the segment of the conduit. Preferably, the reservoir comprises a collapsible wall and the infusion pump further comprises an inflatable bladder juxtaposed with the collapsible wall of the reservoir. The volumetric air pump is in fluid communication with the bladder. The volumetric air pump provides gas to the bladder at a first rate until the select pressure is applied to the reservoir and thereafter provides gas to the bladder at a second substantially uniform rate less than the first rate.
A second aspect of the present invention is an infusion pump including a collapsible bag defining a therapeutic liquid reservoir, the collapsible bag having a resilient tubing defining a reservoir outlet extending therefrom. An inflatable bladder is juxtaposed with the collapsible bag. A rigid housing defines a chamber receiving the collapsible bag and the inflatable bladder and includes a port receiving the resilient tubing. A volumetric air pump is in fluid communication with the bladder to inflate the bladder and thereby apply pressure to the collapsible bag. A valve is operatively associated with the resilient tubing to variably restrict the flow of liquid through the resilient tubing as a function of the pressure applied to the collapsible bag to maintain the substantially constant select pressure in the collapsible bag. The valve preferably prevents the flow of liquid through the resilient tubing until application of the select pressure to the collapsible bag. The valve may consist of a blade and anvil receiving the resilient tubing therebetween. The blade is biased toward the anvil to restrict the flow of liquid through the resilient tubing and the blade is operatively associated with the collapsible bag to bias the blade away from the anvil as pressure within the collapsible bag increases. The volumetric air pump preferably provides gas to the bladder at a first rate until the select pressure sufficient to overcome the biasing force on the blade so as to initially move the arm and the blade away from the anvil is applied to the reservoir and thereafter provides gas to the bladder at a second substantially uniform rate less than the first rate. A detector may be operatively associated with the arm and the volumetric air pump with the detector detecting the initial movement of the arm and sending a first signal to the volumetric air pump. The sensor may further detect if the blade moves more than a select distance from the anvil, at which point a second signal is sent to the volumetric air pump.
A third aspect of the present invention is a method of infusing therapeutic liquid to a patient. The method includes providing a reservoir containing a therapeutic liquid and having a liquid outlet, providing a volumetric air pump in operative association with the reservoir to pressurize the reservoir and maintaining the select pressure in the reservoir so that liquid through the liquid outlet is proportional to the volumetric flow rate of the volumetric air pump. The liquid outlet may comprise a resilient tube and the method may further comprise applying a pinching force to a segment of the resilient tube and decreasing the pinching force as the pressure in the reservoir increases to increase an effective inner cross-sectional area of the segment of the resilient tube. The method may further comprise pressurizing the reservoir by providing a bladder juxtaposed with the reservoir in fluid communication with the volumetric air pump and preventing flow of liquid through the liquid outlet if the pressure in the reservoir is less than the select pressure. The method may further include inflating the bladder at a first rate until the select pressure is reached in the reservoir and thereafter inflating the bladder at a second substantially constant rate less than the first rate.
The infusion pump with servo valve control and method of infusing therapeutic liquid of the present invention provides a highly reliable manner to maintain constant flow rates from the infusion pump over a wide range of flow rates. By directly linking the effective inner cross-sectional area of the outlet to a select pressure applied to the collapsible reservoir, a select pressure at the outlet can be accurately maintained, thereby insuring a substantially constant flow rate by keeping the liquid volumetric flow rate proportional to the volumetric rate of air provided by the volumetric air pump. If the pressure in a line to a patient increases, for example, due to lowering of the pump relative to the point of infusion, pressure will build within the bag which will thereby cause the valve to open further, allowing for a gradually increasing flow rate until the pressure in the bag returns to the select pressure. In this manner, the pump is self regulating. If the valve opens beyond a select amount, indicating a downstream occlusion, the volumetric air pump stops operating, preventing an excessive pressure build up.